It is known in the art to deposit solder paste on a circuit board by screen printing. The screen has to be prepared beforehand and the pattern of deposited paste cannot be changed from one board to the next. For modification or repair of the patterns it is known to use a dispensing needle.
The applicant Micronic Mydata AB has developed a method for jetting of solder paste, i.e. shooting of blobs of solder paste from a distance similar to how ink is jetted in an inkjet printer. The solder paste is pushed out through a narrow nozzle by a plunger in a cylinder and after each shot the cylinder is refilled by a pump.
The paste contains 40-60% by volume of solder balls, typically 20 microns in diameter, and the rest of the volume is solder flux. The flux has a number of functions: making the paste behave as a (thick) liquid, be tacky enough to hold components before and during soldering, protecting the solder balls from oxidation, and removing the oxide from the solder balls and other surfaces during soldering. The main component is a resin, often a natural rosin, which is tacky and also weakly acidic during soldering. There may be an activating compound for removing the oxide and making the solder wet the metal during soldering, often an organic acid and/or a halide-containing compound. There may be other components such as gelling agents to give the desired viscosity to the flux.
The solder paste technology has developed over a long time. The design of solder paste compositions is typically targeted for screen printing, as depicted in FIG. 1A, screen 1011, typically a metal screen with etched holes, is placed flat on the circuit board 1010, and the solder paste 1012 containing balls of solder and flux is scraped (1) over the surface by a knife 1013, so that the holes in the screen are filled and the surplus paste is removed. When the screen is lifted (2) patches of solder paste 1014 remains on the circuit board.
Paste may also be deposited in patches on the circuit board 2014, 2024 by dispensing, FIG. 2A, and jetting, FIG. 2B. In dispensing, the paste 2017 is held in a cylinder 2010 and pushed through a needle 2013 by a plunger 2012 by a well moderated pressure 2015, symbolically drawn as finger force although it may in real life be the force from a piezo element, an electric motor or the like. Dispensed paste may be replaced in the cylinder through a feeding or replenishing tube 2016.
Jetting is similar to dispensing with a cylinder 2020 holding the paste 2021 under a plunger or piston 2022 and a replenishment tube 2026. The needle is replaced by a narrow hole, the nozzle 2023, and the slowly varying pressure 2015 on the plunger 2012 is replaced by high pressure impulses symbolized by hammer blows 2025 and in real life implemented by a piezoelectric, magnetostrictive, thermal, etc. element. Each pressure impulse shoots out a small amount or pellet of paste 2027. Jetting is faster and more flexible than dispensing, but subjects the solder paste to more violent treatment.
One of the problems that all solder application techniques experience and particularly the jetting and dispensing techniques, is the continuous and efficient application of the solder paste through constant use of the application device. More specifically, the solder paste can agglomerate on and stick to surfaces during application due to friction between solder balls and the surfaces of the application components.
FIGS. 8 and 9 show some problems with prior art jetting and dispensing soldering application methods. In FIG. 8 the paste 3054 comprises a matrix 3053, e.g. a flux or a glue, with suspended particles 3052, e.g. spheres of solder, is ejected as a droplet 3056 when the piston 3051 is forcibly pushed down in the cylinder 3010. The viscosity is less in the matrix than in the paste, and there is a risk of segregation between the flux and solder balls. In particular, a moderate pressure inside the cylinder may force the matrix to flow out through the nozzle before the particles. The result is may be a blockage of the nozzle, since the solid fraction of the paste rises in and adjacent to the nozzle. In FIG. 9, a cylinder and nozzle filled with paste is shown in cross-section. The solid content is approximately 45-50% and the viscosity is strongly dependent on the concentration. The particles (solder spheres) are randomly distributed. Locally, the concentration of particles may by higher than the average concentration. Some regions 3073 may by random have a higher density of particles and appear to have a higher viscosity. The dense region 3073 appears almost like a solid grain in the paste and if it enters the nozzle it may clog the nozzle, at least until the random movement of particles has diluted it. The conclusion is that the granular paste may at random temporarily clog the nozzle although there are no clusters in the paste which are larger than the diameter of the nozzle.